Inverse Synthetic Aperture Radar (ISAR) is a technique used to generate two-dimensional resolutions (images) of an object or a target. Unlike many other radar techniques, ISAR may be used to identify a target by imaging descriptive attributes of the target, and therefore enable classification of the target (e.g., type of ship, aircraft, etc.). ISAR processing uses the rotational motion of the target object to extract image points after removing the relative mean motion between a sensor and an object.
Radars typically use some form of waveform modulation as a basis for pulse compression so that the average power of the sensor can be increased without degrading range resolution. Common forms of waveform modulation are binary phase modulation and linear frequency modulation or chirp.
Radars typically use stretch processing when utilizing chirp modulation in a high resolution sensor. Stretch processing performs a range to frequency mapping of the return signal so the range extent of the image space of the target can be translated into a frequency extent in the intermediate frequency (IF) portion of the radar receiver. This is physically accomplished by multiplying the received signal by a dechirp reference signal and then bandpass-filtering the resulting product. The mapping is controlled by the chirp rate and initiation time of the dechirp signal. Stretch processing is typically used for implementing the radar receiver with lower speed analog to digital converters (ADC's) in exchange for reduced range swath coverage. Azimuth swath coverage is controlled by the antenna beamwidth. Pulse compression is achieved by performing a fast Fourier transform (FFT) on the range samples.
Extending the range swath of a stretch type system typically involves increasing the length of the dechirp signal. Thus, this involves greatly expanding the linearity requirements of the dechirp generator and complicates receiver implementation. Increasing the resolution requires increased waveform bandwidth, which again stresses the chirp and dechirp generation implementations. Bandwidth increases can be achieved in stepwise fashion. Prior art includes the concept of transmitting a large bandwidth chirp signal, but using a piecewise dechirp (a ‘sawtooth’ or step chirp type waveform) and then recombining the dechirped segments to obtain the desired total bandwidth in the received signal.
Functionally, single ISAR images have been generated in the radar by positioning the pulse return sample interval at the desired range of the target. With a stretch type radar, this means positioning the dechirp reference signal at the correct range interval as well. The target is typically under track at the time of such imaging in order to remove the relative mean motion between sensor and target. The residual signal effects of rigid body rotational motion of the target can be used to separate out individual pixels in the ISAR image. Such images are typically used to assist in target classification.